Samsung omnia i900 wm6.1, how to connect to a wifi router?

[jnt]

The phone has never been sim locked. There is some kind of 3g support via my carrier, because I can make video calls. But I don't have a data plan. I'm unable to set the wifi connection on the phone so I can use the internet via my router. I was able to wifi connect to the router (the 'connect symbol' was shown on the display) but how I did it, I don't know. The same moment I open a browser, I've tried several, the wifi connection is cut off ( display symbol shows that). Often the browser will hang, or it slowly loads a chosen web page. But it will just work for a short period of time, and I'm pretty sure it uses a gprs connection, because I get all kind of gprs related error messages, and a carrier/operator name I have no knowledge of using, is from time to time shown in the opera browser. If I remove the sim card, I never succeed in downloading a web page. I made so many settings, which I had no knowledge about that at the end nothing worked, and I made a hard reset. I'm looking for an expert, who can give me an easy to read step by step guide, on how to set the phone so it stays router connected when I want it. Maybe the phone is defective, I can't say. Thanks in advance for a guide.